Reusable auto injectors that can be loaded with a container subassembly, such as a syringe, are known in the prior art. For example, U.S. Pat. No. 5,599,309 discloses an auto injector for use with proprietary syringes comprising a drive member that receives the rear end of a plunger of a syringe and a trigger for releasing the drive member such that a medicament can be injected. Drawbacks with prior known device is that once the trigger is cocked there is no safety mechanism to prevent a user from accidentally pushing the trigger and firing the device before the auto-injector has been placed against an injection site.